In general, in a light-emitting element having a double hetero structure, a primary light emitted at a light-emitting layer directs not only toward a front surface of the light-emitting element but also isotropically toward all directions. The primary light directing toward a rear surface of the light-emitting element is absorbed or scattered by a rear-surface electrode or a substrate, and hence does not contribute directly to a light output, which causes the reduction of a light-emitting efficiency.
In order to suppress such a phenomenon as this, there has been developed a technique in which a reflective film formed by at least one layer and reflecting the primary light is arranged between the substrate and the light-emitting layer. However, with this technique, it is difficult to completely reflect the primary light, that is, part of the primary light passes through the reflective film and reaches the substrate, and an excited light is secondarily emitted from the substrate. This excited light from the substrate has a wavelength different from the primary light, and hence, causes a problem that the excited light serves as a noise when used in a sensor for example, possibly having a deleterious effect on reliability of operation of the sensor. Further, in a case of suppressing the excited light from the substrate by absorbing the light, the excited light is accumulated as heat within the light-emitting element, which also has a deleterious effect on the reliability of operation of the sensor.
In view of the facts described above, Patent Literature 1 discloses a technique in which, in a red light emitting diode, a transparent layer having a higher Al composition than that of an active layer is formed between a substrate and the active layer to control an intensity of an excited light generated at the substrate.
Further, Patent Literature 2 discloses a technique in which a reflective layer is disposed between a light-emitting layer and a semiconductor having a composition different from the light-emitting layer, to suppress an excited light generated at the semiconductor from being picked up from a surface.
The purposes of these disclosed techniques are to control or suppress the intensity of the generated exited light, but a peak intensity of the excited light generated at the substrate cannot be sufficiently suppressed. Further, in the case of this method of controlling the excited light, light energy is absorbed, so that heat is accumulated.